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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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SO                                         ALASKAN GLACIER STUDIES
many places is friable schist or slate, or sheeted and jointed gneiss and granite that is readily broken off by weathering or by avalanche erosion; (4) the oversteeping of the valley slopes by long continued glacial erosion, causing cliffs and precipices do"wn which rock fragments are all the time avalanching to the glacier surface; (5) earthquake shattering and shaking, forming rifts in the rock and thus aiding in weathering and in the production of rock avalanches, and earthquake shaking acting directly as a cause for abundant avalanches of snow and rock.
Since the St. Elias Range rises abruptly from near sea level, presenting a steep face toward the sea, the larger glaciers to which the heavy snowfall gives rise easily find their way down to or near sea level, for the snow line even on the seaward face of the mountains is not more than 3000 or 3500 feet, while in sections less exposed to the warm air and rains of the ocean it is no higher than 2000 or 2500 feet. For great glaciers to push their termini down 3000 or 3500 feet below snow line is not difficult. Shorter glaciers fail to do so, and in the Yakutat Bay region there are scores of such glaciers, some ending almost at snow line, others descending part way to sea level. Such glaciers differ but slightly, if at all, from the type of ordinary Alpine glaciers, and in a region of abundant great glaciers attract so little attention, that they have so far received little study, and are for the most part still unnamed.
Of the large glaciers which reach nearly or quite down to sea level there are four distinct types. The first of these, illustrated by Hidden, Fourth, and Black Glaciers, is not greatly different from ordinary valley glaciers, though in the case of the first two, forming a part of a through glacier system. The glaciers of this type are enclosed and terminate in mountain valleys, ending near sea level. The second type of glacier is the tidal glacier, illustrated here by Nunatak (PL V), Turner and Hubbard Glaciers. These may all be parts of a through glacier system, as Nunatak and Hubbard Glaciers certainly are; but below the supply area they have the characteristics of valley glaciers of large size, moving with great rapidity. Instead of ending on the land, however, they terminate in ice cliSs rising out of the sea, and discharging icebergs into it. The third type of glacier, which may or may not be part of a through glacier system, has characteristics of an Alpine valley glacier in its mountain valley, but, being forced beyond the TnmiTit.fl.iTi front, terminates in an expanded foot, or bulb, at the mountain base, where no longer confined between mountain walls. Such a piedmont ice bulb may lie entirely outside the mountains, as on the coastal plain which skirts the St. Elias Range, illustrated by Galiano Glacier and other large glaciers to the southeast of Yakutat Bay (PL I, B), or it may lie in a broader valley within the mountains, as in the case of Variegated Glacier in Yakutat Bay; or better still in the Allen and Miles Glaciers of the Copper River valley. The fourth type is the piedmont ice plateau, of which the Malaspina Glacier is the typical example, made by the union of two or more.piedmont ice bulbs. Malaspina and Bering Glaciers, the largest known examples of this class, are each made by the union of many glaciers; but there are other cases of the same type where only two or three glacier bulbs are united,—for example, the piedmont bulb made by the union of Atrevida and Lucia Glaciers, just west of Yakutat Bay.
In addition to the differences between these four types of glaciers just mentioned there are other notable peculiarities. The tidal glacier maintains activity to its very front, and in all cases observed in this region is crevassed down to the ice cliff; but the other types gradually diminish in activity toward their ends and finally assume almost com-